1. Field of the Invention
The present invention relates to surge protection circuits. In particular, the invention relates to a surge protection circuit for Ethernet networks that carry high-speed digital signals, wherein the circuit provides protection equivalent to that required for outside telephony equipment.
2. Description of the Related Art
Telephone lines typically carry voice and data signals from a remote unit to a local unit. In the preferred environment of the invention, which is a fiber-to-the-curb digital loop carrier system, the remote unit is a remote digital terminal (RDT), and the local unit is an optical network unit (ONU). The ONU distributes digital signals received from the RDT to a plurality of Network Interface Devices (NIDs), which are coupled to single homes or buildings. The NIDs provide an interface between the wiring outside the home or building and the wiring inside the structure serviced by the NID. The NIDs also provide surge protection and a means for disconnecting the inside wiring from the external wiring for testing purposes.
From the NID, digital signals are transmitted to internal electronic devices that use the signals, e.g., computers, telephones, televisions, etc., which are preferably located inside the single home or building. These internal devices are also referred to as customer premises equipment (CPE). The electrical lines between the ONUs and the NIDs typically include twisted pairs of wires. For 10 Base-T Ethernet networks, there is a first twisted pair used for incoming signals and a second twisted pair used for outgoing signals. These twisted pairs are susceptible to voltage and current surges due to lightning strikes or power (50 or 60 Hz) surges. Therefore, surge protection circuits (located in the NID, ONU or both) are coupled to the lines to protect the ONUs, the NID, and the CPE from being damaged by the over-voltage and over-current conditions.
FIG. 1 sets forth presently known surge protection circuit 10. The equipment-side input terminals 22 and 24 of the surge protection circuit 10 are coupled either to the ONU or the CPE, depending on the location of the circuit (either at the ONU or the NiD). The output terminals 18 and 20 are coupled to the outside twisted pair wiring loop between the ONU and the NID, which is exposed to lighting strikes and power fault surges. The surge protection circuit 10 includes a primary shunt protector 26 and a secondary shunt protector 28. The protectors 26 and 28 are coupled together by coordinating resistance devices 30 and 32, which are connected in series with the input and output terminals of the device.
The primary shunt protector 26 is preferably a tip-to-ground and ring-to-ground gas tube protector, or a 3-element gas tube. When there is a surge on the twisted pair wires connected to output terminals 18 and 20, the primary protector 26 fires when the surge reaches the firing voltage of the tube. The primary protector 26 protects against surges of thousands of amps and thousands of volts. It does this by presenting a low-resistance path to ground when it fires, so that the voltage developed across it becomes small after firing. But there are still potentially damaging surges that may pass through the primary protector 26. These may be surges that exceed the DC firing voltage of the protector 26, or they may be AC signals that never reach the firing voltage of the primary protector 26, but which would be damaging to the ONU, the NID, or the CPE.
To provide further protection, the surge protection circuit 10 also includes a secondary protector 28. This secondary protector 28 further reduces the surges that are passed by the primary protector 26. Typically, the secondary protector 28 includes a semiconductor-type shunt voltage clamp, such as a bi-directional thyristor or zener diode. The secondary protector 28 is not capable of surviving the extreme surges that occur if the primary protector 26 does not fire. Thus, without the coordinating resistance devices 30 and 32, the secondary protector 28 would clamp down the voltage such that the voltage would never become large enough to fire the primary protector 26. This happens because the semiconductor-type shunt clamp of the secondary protector 28 fires faster than the gas tube of the primary protector 26, e.g., in nanoseconds as compared to microseconds. Accordingly, if the coordinating resistance devices 30 and 32 are not used, the semiconductor protector 28 takes the entire surge, and thus prevents the gas tube 26 from firing. For this reason, the coordinating resistance devices 30 and 32 are placed in-line between the two protectors 26, 28.
In another known art system, a fuse is used in place of the coordinating resistance devices 30 and 32. In this system, opening of the fuses 30 and 32 by the surge allows the primary protector 26 to fire, but this also eliminates the data path, since the blown fuses are open-circuits and the fuses 30 and 32 must be replaced. One possible solution to fuse replacement is to use a resettable fuse. These devices are commonly used and they function as resistors until the current through them causes the device to reach a critical temperature. At the critical temperature, the resistance increases by several orders of magnitude. After the surge current dissipates, the resistance of the device returns to a low value.
Using resistors or resettable fuses as the coordinating resistance devices 30 and 32, however, causes an undesirable signal loss in the data transmission system at high frequency. Thus, a new surge protection circuit is needed in this field that is particularly well-suited for high frequency data applications.
In accordance with the present invention, a surge protection circuit is coupled to the wires that transport digital signals between a local unit, such as an ONU, and a remote circuit, such as a NID. The surge protection circuit includes a coordinating impedance having a capacitive element connected in series with the twisted pair wires coupling the ONU to the NID. The coordinating impedance couples a primary shunt protector and a secondary shunt protector. Various embodiments of the surge protection circuit comprise coordinating impedance devices including a series connected: capacitor and resistor, capacitor and inductor, and capacitor, inductor, and resistor. By using a capacitive element in the coordinating impedance, the signal loss through the surge protector is significantly reduced for 10 Base-T and 100 Base-T Ethernet systems. Further, the added inductance may become part of an integrated low-pass filter that further reduces signal loss at high frequencies.
As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the spirit of the invention. Accordingly, the drawings and description of the preferred embodiments are to be regarded as illustrative in nature and not restrictive.